Reminds me of a report that got filed in 2007 while I was working for a now sadly defunct LGW based airline operating a Boeing 757. The report detailed a near miss with a microlight over the Isle of Wight at about 16,500ft, about 500ft off the port side I seem to remember. We filed an AIRPROX, did follow up investigation which included speaking to the pilot but never got any conclusions.
It sticks out in my mind because on the first read of the report, I assumed it was on approach to 26L at LGW, so concluded traffic from Redhill. Many lessons learnt including never assume anything!